Category: Data

High-Impact Hydrologic Events and Atmospheric Rivers in California: An Investigation using the NCEI Storm Events Database

April 12, 2017

Two 2016 graduates of the M.S. Applied Meteorology program at Plymouth State University, Klint Skelly (May 2016) and Allison Young (December 2016) advised by CW3E Affiliate Dr. Jason Cordeira, worked collectively on understanding the fraction of floods, flash floods, and debris flows (termed high-impact hydrologic events, or HIHEs) that are associated with landfalling ARs in California.

The HIHE–AR relationship was studied over a 10-water year period from Oct 2004 through Sep 2014 with HIHE reports obtained from the National Centers for Environmental Information (NCEI) Storm Events Database and AR dates obtained from a catalog of landfalling ARs from Rutz et al. (2013). Some detailed results are provided below. More information is contained in a manuscript that was recently published in the AGU Geophysical Research Letters: Young, A. M., K. T Skelly, and J. M. Cordeira, 2017: High-Impact Hydrologic Events and Atmospheric Rivers in California: An Investigation using the NCEI Storm Events Database. Geophys. Res. Lett., 44, doi:10.1002/2017GL073077. click here for personal use pdf file

Key Results: A total of 1,415 HIHE reports in California during the 10-year period of study reduced to 580 HIHE days across the different National Weather Service County Warning Areas (CWAs). A large majority (82.9%) of HIHE days occur over southern California; however, a larger fraction of HIHEs are associated with landfalling ARs across northern California (80.8%) as compared to southern California (41.8%). The 580 HIHE days across the different CWAs, when combined, reduced to 364 unique HIHE days for the state of California. A larger number of HIHE days statewide occur during summer (57.1%) as compared to winter (42.9%). Conversely, a larger fraction of HIHE days associated with ARs occur in winter (78.2%) as compared to summer (25.0%), which corresponds to similar values obtained by Neiman et al., (2008) and Ralph and Dettinger (2012).

Figure caption: Total number of HIHE days per (a) CWA and (b–d) month for (b) all of California, (c) northern California, and (d) southern California. The blue bars and denominator represent the total number of HIHE days, whereas the white hatched bars and numerator represent the total number of HIHE days associated with ARs.

The 580 HIHE days across different CWAs, when combined by region, reduced to 88 unique HIHE days for northern California and 301 unique HIHE days for southern California. A larger number of HIHE days across northern California occur during winter (62.5%) as compared to summer (37.5%), whereas a larger number of HIHE days across southern California occur during summer (60.8%) as compared to winter (39.2%). The fraction of these HIHE days that are associated with ARs is higher over northern California (63.6%) as compared to southern California (39.2%).

This study illustrated that HIHE days contained within the NCEI Storm Events Database for CWAs across California can be attributed to landfalling ARs and their associated precipitation extremes. This attribution is largely valid for HIHE days across northern California in the cold season and not necessarily valid for HIHE days across southern California during the warm season. Approximately 57% of all HIHE days in California occurred during the warm-season, mostly in conjunction with flash floods, and 75% of these HIHE days were not associated with ARs. The composite analysis of flash flood days across California illustrated the climatological warm-season flow pattern for precipitation across southern California and closely resembled the type-IV monsoon synoptic pattern as defined by Maddox et al. (1980). This result motivates additional future work that could focus on the role of the North American monsoon and other non-AR processes that produce HIHEs across California.

Support for this project was provided by the State of California-Department of Water Resources and the U.S. Army Corps of Engineers, both as part of broader projects led by CW3E. Dr. Cordeira and his graduate students at Plymouth State University actively collaborate with CW3E on topics related to atmospheric rivers, such as analyzing, understanding, and forecasting their impacts along the U.S. West Coast.

How Many Atmospheric Rivers Have Hit the U.S. West Coast During the Remarkably Wet Water Year 2017?

April 6, 2017

It has been well established that much of the west coast receives roughly 30-50% of its annual precipitation from landfalling atmospheric rivers. One of the goals of CW3E is to provide timely information on atmospheric rivers and their impacts on water in the West. The analysis presented here is based upon examination of AR conditions on each day from 1 October 2016 through 31 March 2017. Research-based criteria for AR identification have been used, especially the strength of integrated vapor transport (IVT). ARs are also ranked according to a simple scale introduced in 2016 (see inset in the graphic for the scaling).

As would be expected, one reason this winter has been so wet in the west is the large number of landfalling ARs. In addition, a large fraction of these events has been strong, or even extreme, in magnitude, and have caused serious flooding, and incidents like the Oroville Dam spillway issue.

Contacts: F. Martin Ralph, Chad Hecht, Brian Kawzenuk

There have been 45 total atmospheric rivers that have made landfall over the U.S. West coast from 1 October to 31 March 2017. Of the 45 total ARs, 10 have been Weak, 20 have been Moderate, 12 have been Strong, and 3 have been Extreme (Based on IVT magnitude). 1/3 of the landfalling ARs have been “strong” or “extreme”.

The large number of ARs that have made landfall over the U.S. West Coast have produced large amounts of precipitation. The Northern Sierra 8-station index is currently at 83.4 inches, which is just 5.1 inches below the wettest year on record with seven months remaining in the water year. The graphic below, from the California Department of Water Resources, highlights this information.

Odds of Reaching 100% of Normal Precipitation for Water Year 2017 (April Update)

April 6, 2017

Contribution from Dr. M.D. Dettinger, USGS

The odds shown here are the odds of precipitation in the rest of the water year (after March 2017) totaling a large enough amount to bring the water-year total to equal or exceed the percentage of normal listed. “All Yrs” odds based on monthly divisional precipitation totals from water year 1896-2015. Numbers in parenthesis are the corresponding odds if precipitation through March had been precisely normal (1981-2010 baseline).

At the end of a given month, if we know how much precipitation has fallen to date (in the water year), the amount of precipitation that will be required to close out the water year (on Sept 30) with a water-year total equal to the long-term normal is just that normal amount minus the amount received to date. Thus the odds of reaching normal by the end of the water year are just the odds of precipitation during the remaining of the year equaling or exceeding that remaining amount.

To arrive at the probabilities shown, the precipitation totals for the remaining months of the water year were tabulated in the long-term historical record and the number of years in which that precipitation total equaled or exceeded the amount still needed to reach normal were counted. The fraction of years that at least reached that threshold is the probability estimate. This simple calculation was performed for a full range of possible starting months (from November thru September) and for a wide range of initial (year-to-date) precipitation conditions. The calculation was also made for the probabilities of reaching 75% of normal by end of water year, 125%, and 150%, to ensure that the resulting tables of probabilities cover almost the full range of situations that will come up in the future.

[One key simplifying assumption goes into estimating the probabilities this way: The assumption that the amount of precipitation that will fall in the remainder of a water year does not depend on the amount that has already fallen in that water year to date. This assumption was tested for each month of the year by correlating historical year-to-date amounts with the remainder-of-the-year amounts, and the resulting correlations were never statistically significantly different from zero, except possibly when the beginning month is March, for which there is a small positive correlation between Oct-Mar and Apr-Sept precipitation historically.]

Odds of Reaching 100% of Normal Precipitation for Water Year 2017 (March Update)

March 8, 2017

Contribution from Dr. M.D. Dettinger, USGS

The odds shown here are the odds of precipitation in the rest of the water year (after February 2017) totaling a large enough amount to bring the water-year total to equal or exceed the percentage of normal listed. “All Yrs” odds based on monthly divisional precipitation totals from water year 1896-2015. Numbers in parenthesis are the corresponding odds if precipitation through February had been precisely normal (1981-2010 baseline).

At the end of a given month, if we know how much precipitation has fallen to date (in the water year), the amount of precipitation that will be required to close out the water year (on Sept 30) with a water-year total equal to the long-term normal is just that normal amount minus the amount received to date. Thus the odds of reaching normal by the end of the water year are just the odds of precipitation during the remaining of the year equaling or exceeding that remaining amount.

To arrive at the probabilities shown, the precipitation totals for the remaining months of the water year were tabulated in the long-term historical record and the number of years in which that precipitation total equaled or exceeded the amount still needed to reach normal were counted. The fraction of years that at least reached that threshold is the probability estimate. This simple calculation was performed for a full range of possible starting months (from November thru September) and for a wide range of initial (year-to-date) precipitation conditions. The calculation was also made for the probabilities of reaching 75% of normal by end of water year, 125%, and 150%, to ensure that the resulting tables of probabilities cover almost the full range of situations that will come up in the future.

[One key simplifying assumption goes into estimating the probabilities this way: The assumption that the amount of precipitation that will fall in the remainder of a water year does not depend on the amount that has already fallen in that water year to date. This assumption was tested for each month of the year by correlating historical year-to-date amounts with the remainder-of-the-year amounts, and the resulting correlations were never statistically significantly different from zero, except possibly when the beginning month is March, for which there is a small positive correlation between Oct-Mar and Apr-Sept precipitation historically.]

Current Winter Setting a New California-Wide Record Precipitation Accumulation

March 7, 2017

Fueled by a string of strong atmospheric rivers (ARs), California’s current winter-to-date accumulated precipitation has hit a new record high level, eclipsing the previous record set during the strong El Niño winter of 1982-83.

The winter began with an unusual early season AR, which contributed 6% of normal annual California-wide precipitation over the period Oct 14-17. Strong AR activity continued in Jan and Feb 2017, with exceptionally strong precipitation Jan 8-10, which produced 14% of normal statewide annual precipitation in just three days and reached R-cat 4 intensity. (R-cat levels measure intense precipitation events; a fuller description of R-cat levels and this event can be found here). The AR during Feb 7-9 produced 9.5% of total annual California precipitation. Together, the latter two AR events produced nearly a quarter of an entire normal year’s precipitation in just 6 days, with each event including extreme intensity AR landfalls in the state.

The figure below shows the water year (Oct 1st – the following Sep 30th) that holds the record for maximum precipitation in California accumulated since the beginning of October for each day of winter. The current water year, 2017, broke the old record in early February and has continued to be the record-holder up to the current date (first week of March). Currently, 1982-82 holds the record for the maximum state-wide accumulated precipitation at the end of May in observations that go back to 1948. The accumulation so far this year is above the pace of 1982-83, but 1982-83 received a significant amount of precipitation in March and early May.

This figure shows California statewide accumulated precipitation estimated from 96 stations distributed across the state, but similar results are seen in the “Eight Station Index”, which uses eight stations in the Sierra Nevada selected for their importance to the state’s water supply. The eight station index is likewise currently at new record levels of accumulated winter precipitation, superseding the previous record-holding winter of 1982-83.

The southern portion of the state, including the greater Los Angeles region and San Diego county, are unusually wet so far this winter but not at record breaking levels. For instance, the Los Angeles region received substantially more precipitation in 2005, which led to widespread flooding, infrastructure damage, and several deaths.

The record-breaking precipitation has led to high values of snow cover, as shown by the yellow line (winter of 2016-2017) below. In the central and southern Sierra Nevada, current values are almost twice what is seen at the typical peak of snow accumulation on April 1st, and significantly above the high values seen during the El Niño winter of 1997-98 (dashed blue line). Snow is an important component of California’s water supply, since it holds the precipitation from intense winter storms, releasing the water more slowly via snow melt.

Summary of the ARs that impacted the U.S. West Coast over the past week

Landfalling AR brought weak-to-moderate AR conditions to portions of Southern CA for ~24 hours between 27 and 28 February

>6 inches of precipitation fell over the high elevations of San Diego County with lower elevations receiving 1.5–4 in.

The San Diego River rose to ~14.15 feet at 2 am 28 Feb, 2.8 feet above flood stage, and the 3rd highest peak all time

The heavy precipitation led to several road closures, multiple mudslides, hotel evacuations, and flooded businesses

Click IVT or IWV image to see loop of GFS analysis

Valid 0000 UTC 26 Feb – 0600 UTC 28 Feb 2017

Three ARs expected to make landfall over the U.S. West Coast over the next ten days

The first AR is expected to make landfall over the Pac NW ~1800 UTC 2 March 2017 with weak strength (IVT=250–500 kg m-1s-1). Weak AR conditions may propagate over N CA. prior to dissipation.

A second AR is expected to make landfall over N CA. at ~0000 UTC 5 March 2017. Coastal areas of N CA may see several hours of moderate strength AR conditions.

Long range forecasts indicate the potential for a third weak AR during 8-10 March 2017, however there is large uncertainty in the models beyond forecast day 5.

Large scale pattern beyond forecast day 9 indicates the potential for a return to active AR landfall conditions over the Pac NW
Highest precipitation and impacts from these events is predicted to be over the Olympic and Cascade Mtns. in WA and Coastal Mtns. In NW CA.

Summary of the ARs that impacted the U.S. West Coast over the past week

Three separate ARs made landfall and impacted the U.S. West from 14–21 February 2017

Over 20 inches of precipitation fell over some of the high elevations of the West Coast

There were 291 total storms reports made to NOAA NWS during the three ARs

A summary of the ARs and their impacts are discussed in this post event summary

SSMI Integrated Water Vapor (IWV)

Valid 14-21 Feb 2017

The first AR made landfall between 18 UTC (10 AM PST) 14 February and 00 UTC 15 February (4 PM PST 14 Feb) over the Pacific Northwest before propagating southward over California

Maximum IVT at the Coast was between 800 and 1000 kg/m/s, which is considered a strong AR

Some locations experienced AR conditions for up to 42 hours during this event

Note: The strength of AR conditions noted on this summary was determined based on 6-hourly NCEP GFS analysis periods and observed IVT magnitudes may have been higher at specific locations along the coast

The second AR, which developed in conjunction with a mesoscale frontal wave, made landfall ~6 UTC on 17 February (4 PM PST 16 Feb) over Southern CA

Some locations experienced AR conditions for up to 24 hours during this event

Note: The strength of AR conditions noted on this summary was determined based on 6-hourly NCEP GFS analysis periods and observed IVT magnitudes may have been higher at specific locations along the coast

The third AR made landfall at ~6 UTC on 20 February (4 PM PST 19 Feb) over the Northern Ca

Maximum IVT at the Coast was between 700 and 800 kg/m/s, which is considered a moderate strength AR

Some locations experienced AR conditions for up to 42 hours during this event

Note: The strength of AR conditions noted on this summary was determined based on 6-hourly NCEP GFS analysis periods and observed IVT magnitudes may have been higher at specific locations along the coast